1. Field of the Invention
The present invention relates to a fluorescent lamp starter, especially to a starter for starting a fluorescent lamp provided with electrodes using a semiconductor switching element.
2. Description of the Related Art
Conventionally, a glow-starter has mainly been used as a fluorescent lamp starter. However, the glow-starter has problems in that a long time is required to start the fluorescent lamp and the life of the glow-starter itself is short, and the like.
Recently, a starter using a semiconductor switching element has been developed in order to overcome the above problems. However, such a starter requires a high production cost, which hinders the expansion of the practical use thereof. Therefore, there has been an increased demand for an economical starter using a switching element.
Japanese Laid-Open Patent Publication No. 3-252096 discloses a fluorescent lamp starter using a semiconductor switching element as shown in FIG. 5. This conventional fluorescent lamp starter includes an AC power source 101, a ballast 102, a fluorescent lamp 103 having a pair of electrodes 104 and 105, a thyristor 112, a Zener diode 113, four resistances 108, 109, 114, and 115, and two capacitors 110 and 119.
An end of the electrode 104 is connected to the AC power source 101 via the ballast 102. An end of the electrode 105 is also connected to the AC power source 101. The fluorescent lamp 103 is connected to a series circuit which has the diode 106, the resistance 109, and the collector and emitter of the transistor 107, on the opposite side of the AC power source 101. The base of the transistor 107 is connected to the diode 106 via the resistance 114. A control voltage supply means for controlling the thyristor 112, which has the resistance 108 and the capacitor 110 is connected between the diode 106 and the emitter of the transistor 107. The thyristor 112 is connected between the base and the emitter of the transistor 107. The resistance 115 and the Zener diode 113 are connected between the gate of the thyristor 112 and an end of the capacitor 110 on the side of the resistance 108.
Next, the operation of the above conventional starter will be described.
If the AC power source 101 is turned ON, a current is applied between the base and the emitter of the transistor 107 via the resistance 114 when the cycle of the power source voltage is positive, thereby allowing electrical conduction between the collector and the emitter of the transistor 107. As a result, a preheat current is applied from the AC power source 101 to the ballast 102, the electrode 104, the diode 106, the resistance 109, the transistor 107, and the electrode 105. Every time the preheat current is applied so as to correspond to a half wave of the positive cycle of the power source voltage, the electrodes 104 and 105 of the fluorescent lamp 103 are preheated, and the capacitor 110 of the control voltage supply means is charged via the resistance 108. When a voltage across either end of the capacitor 110 exceeds a Zener voltage of the Zener diode 113, the current is applied to the gate of the thyristor 112 via the resistance 115, so that the thyristor 112 enters a conductive state, and the transistor 107 is turned to an OFF-state. At this time, the current is prevented from being applied to the ballast 102, so that a pulse voltage is generated at the ballast 102 having inductance, thereby starting the fluorescent lamp 103.
However, such a conventional starter has disadvantages as described below. For turning ON the thyristor 112, the voltage across the capacitor 110 should exceed the total voltage of the Zener voltage of the Zener diode 113 and the voltage between the gate and the cathode of the thyristor 112. It is difficult to maintain a fixed time period from the time at which the power source is turned ON to the time at which the thyristor 112 is turned ON according to the conventional starter. The reason is that the Zener voltage of the Zener diode 113 and the capacitance of the capacitor 110 are likely to deviate from the design value, and fluctuate depending on the environment. Therefore, it is very difficult to generate the pulse voltage from the ballast 102 with a constant timing. In addition, when the voltage across either side of the fluorescent lamp 103 is not sufficiently large, the transistor 107 is turned to an OFF-state. As a result, the pulse voltage is not sufficiently generated at the ballast 102, so that the fluorescent lamp 103 remains not to burn.